@Banzai
Are you a fan of Saving Private Ryan? Perhaps you remember the line, "Careful not to step in the ********?" In all honesty, you will never hear a truly unbiased argument as to which card is better, everyone has his/her favorite company, and they are very adamant about which is better. Let me give you some insight, as I had the same decision as you a couple of weeks ago. I ended up choosing the 8800 GTS 640MB. Now the card itself is a great card, runs nicely, runs coolly, and can be overlcocked easily to much higher speeds, I run it at 625 core and 900 memory without even entering the yellow areas of danger. I also, having owned ONLY ATI until this card that being for 3 different cards, prefer NVIDIA's control panel much more than CCC for ATI, as I think the NVIDIA control panel is just designed better and offers more. As to how it performs? It performs great, except not amazingly well in Company of Heroes compared to my old 1950 XT, though I am starting to think that is a CPU issue and I plan to try overclocking as most strategy games are very CPU dependent. The image quality is great and so are the frame rates. The truth is that some companies just run some games better than than others.
However, it seems to me that if you look at some previews of ATI's 7.8 drivers, that their drivers are ready to go pretty far and it might end up being better than the GTS pretty much overall. By how much, idk. Does this mean the GTS is a bad performer? Hardly. Do I think that the 2900 will eventually be better overall? Yes, I do, and it makes me slightly regret my purchase. But as of right now, they are a toss up, and either one will do darn well. However, I recommend the GTS for you. You have an nForce mobo, which I don't and I wish I did, so you get some advantages such as tuning your whole system as one. Also, post your PSU specs and brand, cause I have a 500 watt Ultra and I can run the GTS overclocked with no problems at all. You might be able to save yourself some money, cause while I think you could prob run the GTS on your current PSU, you prob wouldn't be able to run the XT, or at least overclock it.
Lastly, and this is the most most important fact. If you are simply going to upgrade next summer, it doesn't really matter. The general consensus right now seems to be that the second generations of DX10 cards will be much more powerful anyway. Taking that into effect, buy the cheapest for now, save yourself some money and then spend it on a second generation card later. You won't miss out on much as DX10 prob won't be into full swing until the beginning to middle of next year anyway.